1. Field of the Invention
The present invention relates to an LED printing device and, more particularly, to a high resolution LED array bar.
2. Brief Description of Related Developments
It is common to use light emitting diode (LED) bars in printing devices. LED bars provide reliable and controllable light sources. The bars generally comprise a plurality of light sources, i.e., pixels that can be activated and deactivated (pulsed) to emit short bursts of light at a high rate of speed. Each light burst is used to create a particular portion of a printed symbol or character. The more often a pixel is pulsed, the more often a symbol or character portion will be imaged, thus providing greater detail and higher resolution printing. Therefore, for the printing to be completed within a commercially reasonable time with high resolution, it is necessary to have a high rate of pulsing.
LED bars are manufactured in different segment, or chip, sizes. Segment size depends on the number of pixels within the segment. Two popular numbers of pixels per segment are 64 pixels and 128 pixels. At 424.26 spot per inch (SPI) these segments would be 3.832 and 7.663 mm respectively. The respective lengths are determined by dividing the number of pixels by the spot per inch requirement and converting the quotient to millimeters. For example:             64      ⁢              (        pixels        )            xc3x97              1                  424.26          ⁢                      xe2x80x83                    ⁢                      (            spi            )                                =                  .1509        ⁢                  xe2x80x83                ⁢        in        xc3x97        25.4        ⁢                  xe2x80x83                ⁢                  mm          in                    =              3.832        ⁢                  xe2x80x83                ⁢        mm                        128      ⁢              (        pixels        )            xc3x97              1                  424.26          ⁢                      xe2x80x83                    ⁢                      (            spi            )                                =                  .3017        ⁢                  xe2x80x83                ⁢        in        xc3x97        25.4        ⁢                  xe2x80x83                ⁢                  mm          in                    =              7.663        ⁢                  xe2x80x83                ⁢        mm            
The technologies that create linear arrays of LED""s, composed of discrete chips placed side-by-side, have evolved to where 600 SPI densities are easily achievable. In fact, this density is found in most printers using LED bars. Higher densities are also possible, and a 1200 SPI bar is on the market.
Evaluation of a 1200 SPI bar revealed an inconsistent pitch. The distance between adjacent pixels on different chips was large by more than 4.3 xcexcm or 20% of the pitch. (see FIG. 1) This much error causes undesirable banding on prints. Clearly, the technology that creates LED""s has improved to where 1200 SPI LED""s are possible, but the technology that places the chips has remained at 600 SPI.
Five design rules govern the creation of true 1200 SPI arrays. State-of-the-art arrays, represented by the evaluated bar, fail to meet all five. The rules are: (1) Emitters can not be too large. Large emitters have optical and electrical crosstalk. (2) Emitters can not be too small. Small emitters inefficiently generate light so require high current and produce high temperatures. (3) Emitters cannot be too close to the chip edge. Close emitters develop an infant mortality caused by fractures created when the chip is diced from the wafer. (4) The gap between chips can not be too small. Small gaps give a high probability that a chip will contact its neighbor and fracture during placement into the array. Furthermore, the gap allows thermal expansion. If chips contact during expansion, they fracture or break the adhesive. (5) The pitch must be consistent or else banding occurs.
Using existing practices, rules (1) and (2) are met as evidenced by the chips of the evaluated bar and by other experimental chips. Chips can be made of viable 10.5 xcexcm width LED""s. Rules (3), (4), and (5) remain problematic though. They are mutually exclusive. Chips can be diced no closer than 5 xcexcm from the emitter. Placement is no better than xc2x11 xcexcm for engineering work and closer to xc2x12.5 xcexcm for production work. So, 1200 SPI chips can be placed on-pitch as shown in FIG. 2 or over-pitch as shown in FIG. 3. On-pitch yields a gap of 0.7 xcexcm. This exceeds even engineering accuracies so is impractical. The smallest over-pitch yields a spacing of 25.5 xcexcm which is 4.3 xcexcm greater than the ideal pitch of 21.2 xcexcm. The evaluated bar uses it, but of course, with the defect.
Thus, it would be helpful to be able to form a 1200 SPI LED array with a consistent pitch while minimizing the array size and distance between adjacent chips.
The present invention is directed to a method of forming a high resolution LED array. In one embodiment the method comprises providing a plurality of LED chips to form the LED array. An electrode of an LED located at each end of each chip is inward biased by a predetermined amount. The size of each LED chip is reduced by removing, at each end of each chip, an amount of chip material substantially equal to the predetermined amount. The array is formed by placing each chip end to end with a gap between each chip, wherein the gap is suitably large for placement accuracies in a consistent pitch of approximately 21.2 xcexcm is maintained between each LED on each chip.
In another aspect, the present invention is directed to a high resolution LED printbar. In one embodiment the high resolution LED printbar comprises a plurality of LED chips butted together with a gap between adjacent LEDs to form an array. Each LED chip generally comprises a plurality of LEDs where each LED is adapted to generate an emitted light. A center electrode extends from each LED and is adapted to electrically connect the LED to a wired bond pad. The center electrode is generally positioned over an emitting side of the LED and a centroid of light from each LED is centered over the LED. An LED at each end of the chip has an electrode that is inward biased over each respective end LED. A centroid of emitted light from each end LED is positioned closer to an outer edge of the chip.